If you want serious optimizations - then Gentoo is your choice. But seriously, there won’t be any serious difference between distributions. What really matters here are DEs and browsers. I would recommend some kind of lightweight window manager like i3 or dwm. If you do not want to configure everything yourself, then your choice is lxde/lxqt. Also, you can use distros without systemd (void, artix, devuan, gentoo etc), but that does not matter that much.